Changeset 190882 in webkit for trunk/Source/JavaScriptCore/runtime/JSStringBuilder.h

Timestamp:

Oct 12, 2015, 12:44:52 PM (10 years ago)

Author:

[email protected]

Message:

"A + B" with strings shouldn't copy if A or B is empty.
<​https://webkit.org/b/150034>

Reviewed by Anders Carlsson.

Source/JavaScriptCore:

• runtime/JSStringBuilder.h:

(JSC::jsMakeNontrivialString):

• runtime/Lookup.cpp:

(JSC::reifyStaticAccessor):

• runtime/ObjectPrototype.cpp:

(JSC::objectProtoFuncToString):

Source/WTF:

Add a fast path to WTF's operator+ magic for concatenation of two strings where
one of them is empty. In that case, try to avoid allocation altogether by returning
the non-empty string.

Spotted this while analyzing memory peaks during page load; it turns out we were
duplicating whole text resources (JS, CSS) at the end of decoding, below
TextResourceDecoder::decodeAndFlush(). That function effectively does:

return decode() + flush();

Very often, flush() returns an empty string, so due to the naive operator+,
we'd allocate a new StringImpl of length (decode().length() + flush().length())
and copy the return value from decode() into it. So silly!

Had to make the tryMakeString() machinery use String as a return type instead of
RefPtr<StringImpl> to make all the right overloads gel. StringTypeAdapter templates
are now required to provide a toString() function.

• wtf/text/StringConcatenate.h:

(WTF::StringTypeAdapter<char>::toString):
(WTF::StringTypeAdapter<UChar>::toString):
(WTF::StringTypeAdapter<Vector<char>>::toString):
(WTF::StringTypeAdapter<String>::toString):
(WTF::tryMakeString):
(WTF::makeString):

• wtf/text/StringOperators.h:

(WTF::StringAppend::operator String):

• wtf/text/StringView.h:

(WTF::StringTypeAdapter<StringView>::toString):

File:

• trunk/Source/JavaScriptCore/runtime/JSStringBuilder.h (modified) (1 diff)

trunk/Source/JavaScriptCore/runtime/JSStringBuilder.h

@@ -128 +128 @@
 inline JSValue jsMakeNontrivialString(ExecState* exec, const StringType& string, const StringTypes&... strings)
 {
-    RefPtr<StringImpl> result = WTF::tryMakeString(string, strings...);
+    String result = WTF::tryMakeString(string, strings...);
     if (!result)
         return throwOutOfMemoryError(exec);
-    return jsNontrivialString(exec, result.release());
+    return jsNontrivialString(exec, result);
 }